Pictures don't turn out or show much, but after much messing with the screens, there is a channel shaped piece of plastic top and bottom. The top one is short enough to slide and have room to lift the screen. The bottom piece is the full length of the window. If the bottom one was put in the opposite way the screen would come out past it, but the taller edge is in the way. I think 20 years ago the bottom plastic piece was probably flexible enough to slide out from under the screen for cleaning and put back in wrong. Not sure but I am going to try and get it out of there, hopefully it won't break but if it does I can get enough of it back in to support the screen. I'm sure that finding new channel would be difficult. Thanks for the advice. Keith